1. Field of the Invention
The application relates to a spread-spectrum clock generator, and more particularly, to a spread-spectrum clock generator capable of adjusting a frequency shift of an output frequency signal.
2. Description of the Prior Art
A conventional spread-spectrum clock generator can generate a spread-spectrum clock signal by using an analog or digital spread-spectrum clock generator. The bandwidth design for the analog spread-spectrum clock generator is limited by the spread-spectrum frequency standard and extra circuit layout is required to achieve the effects of spread spectrum. The analog spread-spectrum clock generator also requires a phase-locked loop to accomplish frequency synthesizing operations with high-resolution. Therefore, the level of spread spectrum easily varies with the processes. Even though a digital spread-spectrum clock generator includes a phase-locked loop, it still generates a spread-spectrum clock signal without passing a feedback route within the phase-locked loop. Therefore, the generated spread-spectrum clock signal easily has a frequency shift.
Please refer to FIG. 1, which is a schematic diagram of a spread-spectrum clock generator 10 according to the prior art. The spread-spectrum clock generator 10 includes a frequency synthesizer 100 and a triangle-wave generator 102. The frequency synthesizer 100 includes a first frequency divider 104, a phase detector 106, a loop filter 108, a voltage controlled oscillator 110, a second frequency divider 112, a third frequency divider 114 and a first sigma-delta modulator 116. The first frequency divider 100 is used for dividing a reference frequency fin to generate a reference signal Sin. The phase detector 106 is coupled to the first frequency divider 104 for generating a phase difference signal Sph according to the reference signal Sin and a first feedback signal Sfb. The loop filter 108 is coupled to the phase detector 106 for generating a wave filtered voltage signal Sflt according to the phase difference signal Sph. The voltage controlled oscillator 110 is coupled to the loop filter 108 for generating a voltage output signal Svo according to the wave filtered voltage signal Sflt. The third frequency divider 114 is coupled between the voltage controlled oscillator 110 and the second frequency divider 112 for dividing a frequency of the voltage output signal Svo to generate an output frequency signal Sof. The second frequency divider 112 is coupled between the phase detector 106 and the third frequency divider 114 for generating the first feedback signal Sfb according to the output frequency signal Sof. The first sigma-delta modulator 116 is coupled between the second frequency divider 112 and the triangle-wave generator 102 for controlling a frequency divided ratio T1 of the second frequency divider 112. The triangle-wave generator 102 is coupled to the first sigma-delta modulator 116 of the frequency synthesizer 100 for generating a triangle-wave signal Stw according to a frequency controlled signal ftw.
In the frequency synthesizer 100, the second frequency divider 112 has an integer operation which is controlled by the first sigma-delta modulator 116, so that the frequency divided ratio T1 of the second frequency divider 112 is a time-variant integer and an average of the frequency divided ratio T1 becomes a non-integer value, to accomplish frequency synthesizing operations with high-resolution. After a triangle wave generated by the triangle-wave generator 102 is outputted to the frequency synthesizer 100 with high-resolution, the spread-spectrum requirement is finished. It is possible for the frequency synthesizer 100 to cause the gain change in the environment with different frequency, voltage, temperature and so on. The outputted frequency signal may become unbalanced after performing spread-spectrum operations. Therefore, how to reduce the frequency shift of the outputted frequency signal after performing spread-spectrum operations due to the different environmental factors becomes a goal for manufacturers.